text: RNA is less stable in the cell, and also more prone to nuclease attack experimentally. As RNA is generated by transcription from DNA, the information is already present in the cell's DNA. However, it is sometimes desirable to sequence RNA molecules. While sequencing DNA gives a genetic profile of an organism, sequencing RNA reflects only the sequences that are actively expressed in the cells. To sequence RNA, the usual method is first to reverse transcribe the RNA extracted from the sample to generate cDNA fragments. This can then be sequenced as described above.
  The bulk of RNA expressed in cells are ribosomal RNAs or small RNAs, detrimental for cellular translation, but often not the focus of a study. This fraction can be removed in vitro, however, to enrich for the messenger RNA, also included, that usually  of interest. Derived from the exons these mRNAs are to be later translated to proteins that support particular cellular functions. The expression profile therefore indicates cellular activity, particularly desired in the studies of diseases, cellular behaviour, responses to reagents or stimuli. Eukaryotic RNA molecules are not necessarily co-linear with their DNA template, as introns are excised. This gives a certain complexity to map the read sequences back to the genome and thereby identify their origin.
  For more information on the capabilities of next-generation sequencing applied to whole transcriptomes see: RNA-Seq and MicroRNA Sequencing.